Organic light-emitting devices comprising an organic compound have been considered to be useful for an economical, solid emission type light-emitting device having a large emission area such as a full color display device and a writing light source array, thereby having been actively studied in recent years. The organic light-emitting device generally comprises a couple of electrodes and one or more organic layers containing a light-emitting layer disposed between the electrodes. When to the organic light-emitting device is applied a voltage, electrons are injected from a negative electrode and holes are injected from a positive electrode to the organic layers. The electrons and the holes are recombined in the light-emitting layer and energy is converted into light while an energy level is turned from a conduction band to a valence band, whereby the organic light-emitting device emits light.
The conventional organic light-emitting devices require high applying voltage for light emission and are poor in luminance and light-emitting efficiency. Some proposals have been provided to overcome the problem in recent years, for example, an organic light-emitting device comprising organic thin layers of a vapor-deposited organic compound has been disclosed in Applied Physics Letters, 51, 913 (1987). This organic light-emitting device has a bilayer lamination structure where an electron-transporting layer and a hole-transporting layer are laminated between the electrodes, thereby exhibiting more excellent light-emitting properties than that of devices having a single-layer structure. This organic light-emitting device uses a low molecular weight amine compound as a hole-transporting material and 8-quinolinol aluminum complex (Alq) as an electron-transporting, light-emitting material to emit a green light. After this disclosure, various organic light-emitting devices comprising the vapor-deposited organic thin layers have been developed as disclosed in Macromolecularly Symposium, 125, 1 (1997) and references therein, etc. However, such organic light-emitting devices are inferior in the light-emitting efficiency to inorganic LED devices and fluorescent tubes, thereby being far from practicable.
Most of the conventional organic light-emitting devices are such that utilizes singlet excitons generated in the organic light-emitting material to provide fluorescence. According to simple mechanism in quantum chemistry, ratio of the singlet excitons providing fluorescence to triplet excitons providing phosphorescence is 1/3 in an excitation state. Therefore, the organic light-emitting device providing fluorescence can practically utilize only 25% of the excitons, thereby inevitably being poor in the light-emitting efficiency.
Under such circumstances, phosphorescent light-emitting devices using a phenylpyridine-iridium complex have been disclosed in Appl. Phys. Lett., Vol. 75, Page 4 (1999), Jpn. J. Appl. Phys., Vol. 38, Page L1502 (1999), etc. in recent years. The light-emitting efficiency of the phosphorescent light-emitting device is two to three times higher than those of the conventional fluorescent light-emitting devices. However, the light-emitting efficiency of the phosphorescent light-emitting device is lower than theoretical light-emitting efficiency, and thus, the light-emitting efficiency has been desired to be further improved to put the phosphorescent light-emitting device into practical use. Further, the phosphorescent light-emitting device has been required to be further improved with respect to the durability because it is inferior therein to the conventional fluorescent light-emitting devices.
Although the organic layer of the organic light-emitting device may be formed by a vapor deposition method, a sputtering method, a CVD method, a PVD method, an applying method using a solvent, etc., the organic layer is desirably formed by a wet film-forming method (a coating method) such as the applying method from the viewpoints of simplification of production processes, reduction of production costs, improvement of workability, application to a flexible device having a large emitting area such as a back light and an illuminated light source, etc. In the known phosphorescent light-emitting devices mentioned above, the organic layer composed of the low molecular weight compound is formed by a dry film-forming method such as a vapor deposition method. Thus, the devices are inevitably deteriorated by crystallization of the low molecular weight compound to require high production costs and to be poor in producibility.
Organic light-emitting devices comprising an organic layer of a high molecular weight compound formed by a wet film-forming method have been disclosed and poly(p-phenylenevinylene) that emits a green light (Nature, Vol. 347, Page 539, (1990)), poly(3-alkylthiophene) that emits an orange-red light (Jpn. J. Appl. Phys., Vol. 30, Page L1938 (1991)), polyalkylfluorene that emits a blue light (Jpn. J. Appl. Phys., Vol. 30, Page L1941 (1991)), etc. have been known as the high molecular weight compound. Further, Japanese Patent Laid-Open No. 2-223188 has disclosed a method where a low molecular weight compound is dispersed in a binder resin and formed into a layer by a wet applying method. However, the organic light-emitting device comprising the organic layer formed by the wet method is such that utilizes the singlet excitons, thereby being still disadvantageous in a low light-emitting efficiency.